As electronic devices and other electrical apparatuses increasingly become portable, advances must be made in energy storage systems to enable such portability. Indeed, it is often the case with current electronics technology that the limiting factor to portability of a given device is the size and weight of the associated energy storage device. Obviously, a small energy storage device may be fabricated for a given electrical device, but at the cost of energy capacity. Conversely, a long-lasting energy source can be built, but it is then too large to be comfortably portable. The result is that the energy source is either too bulky, too heavy, or it doesn't last long enough. The main energy storage device used for portable electronics is the electrochemical battery cell, and less frequently, the electrochemical capacitor.
Numerous different battery systems have been proposed for use over the years. Early rechargeable battery systems included lead-acid, and nickel-cadmium (Nicad), each of which have enjoyed considerable success in the marketplace. Lead-acid batteries, because of their ruggedness and durability, have been the battery of choice in automotive and heavy industrial applications. Conversely, Nicads have been preferred for smaller or portable applications. More recently, nickel metal hydride systems (NiMH) have found increasing acceptance for both large and small applications.
Notwithstanding the success of the aforementioned battery systems, other new batteries are appearing on the horizon which offer the promise of better capacity, better power density, and longer cycle life as compared with the current state of the art. The first such system to reach the market is the Lithium ion battery, which is already finding its way into consumer products. Lithium polymer batteries are also receiving considerable attention, though have not yet reach the market.
Lithium batteries in general include a positive electrode fabricated of a transition metal oxide material, and a negative electrode fabricated of an activated carbon material such as graphite or petroleum coke. New materials for both electrodes have been investigated intensely because of their high potential gravimetric energy density. To date, however, most of the attention has been focused on the transition metal oxide electrode.
Activated carbon materials are routinely prepared by using difunctional monomers as polymer precursors. Examples of such precursors include resins of furfuryl alcohol, phenol, formaldehyde, acetone-furfural, or furfural alcohol-phenol copolymer. Other precursors include polyacrylonitrile and rayon polymers, as disclosed in Jenkins, et al, Polymeric Carbons-Carbon Fibre, Glass and Char, Cambridge University Press, Cambridge, England (1976). These precursors are then subjected to a process of curing and carbonizing, usually very slowly, and at temperatures of up to 2,000.degree. C. Two major steps are involved in these processes: (1) synthesis of polymer precursors from difunctional monomers via wet chemistry; and (2) pyrolysis of the precursors. The method typically results in a relatively low overall yield due to the two step process. For example, conventional processing of polyacrylonitrile typically yields only about 10% of a usable carbonaceous material. Further, many impurities may be incorporated into the carbonaceous material, deleteriously effecting the electrochemical properties.
Accordingly, there exists a need for an improved, carbon material for use in electrochemical cell applications. The material should be easily manufactured in a simple, high yield method.